Method and apparatus for short circuit welding

ABSTRACT

A method and apparatus for short circuit welding includes providing welding power suitable for short circuit welding, sensing the stick out length, and adjusting the welding speed, such as wire feed speed or travel speed, adjusting a welding parameter, or adjusting thee gas mixture in response thereto. Stick out is preferably determined by measuring a welding parameter, and performing an FFT on the parameter, and then calculating stick out, in one embodiment. Stick out can be either CTTWD or CPTPD. The system can determine when a short is about to clear by calculating a value V c  defined by V c =d/dt(kl*dp/dt), and comparing V c  to a V threshold , which varies in response to welding cycle history.

FIELD OF THE INVENTION

The present invention relates generally to the art of More specifically,it relates to short circuit welding.

BACKGROUND OF THE INVENTION

There are many types of welding power supplies and welding processes.One welding process is referred to as short circuit transfer welding.Short circuit transfer welding generally consists of alternating betweenan arc state and a short circuit, non-arc state. During the arc statethe wire melts, and during the short circuit state the metal furthermelts and the molten metal is transferred from the end of the wire tothe weld puddle. The metal transferred in one cycle is referred toherein as a drop, regardless of the size or shape of the portion ofmetal that is transferred.

Short circuit transfer welding has many advantages, such as shorter arclength and less melting of the base plate. However, short circuittransfer welding has disadvantages, such as increased spatter.

Both the power source topology and the control scheme must be consideredwhen designing a short circuit transfer welding power source. The powertopology used must be fast enough to have a timely response to thechosen control scheme. The control should address three considerations:First, arc length must be properly controlled. Second, the burn-off (ormass deposition) rate must be appropriately controlled. Inappropriateburn-off rate will result in increased spatter. Third, spatter is alsocaused by too much power when the short is cleared, i.e., the transitionfrom a short circuit to an arc. Thus, the power or current when theshort clears must also be controlled. Also, when the short is about toclear must be detected. Some prior art patents do not teach control ofthe short circuit transfer welding process on a short circuit by shortcircuit basis. Such a control will provide more precise control of thewelding process and will help to reduce spatter.

One common prior art power source topology uses secondary switchers tocontrol the output. While these may provide fast control, they may berelatively expensive or have insufficient peak current capacity. Also,switching high current may increase reliability problems and switchinglosses. Examples of patents that have secondary switchers include: U.S.Pat. No. 4,469,933, entitled Consumable Electrode Type Arc Welding PowerSource, issued Sep. 4, 1984; U.S. Pat. No. 4,485,293, entitled ShortCircuit Transfer Arc Welding Machine, issued Nov. 27, 1984; U.S. Pat.No. 4,544,826 entitled Method and Device For Controlling Welding PowerSupply to Avoid Spattering of the Weld Material, issued Oct. 1, 1985;U.S. Pat. No. 4,717,807, entitled Method and Device For Controlling aShort Circuiting Type Welding System, issued Jan. 5, 1988.

The control scheme in many prior art power supplies uses arc voltage todetermine if arc length is proper. Typically, if the arc voltage is lessthan a setpoint, the arc length is determined to be too short, and ifthe arc voltage is greater than the setpoint, arc length is determinedto be too long. The output current is controlled to either increase ordecrease the amount of metal melted per short-arc cycle, thuscontrolling the arc length. Some prior art short circuit transferwelding patents taught control of the mass deposition (burn-off) rate bycontrolling the welding power by “totalizing” the energy delivered tothe arc. Arc or welding power is a function of arc current and arcvoltage.

However, the burn-off rate on a short-by-short basis (i.e. for any givenshort circuit transfer welding cycle) is largely independent of arcvoltage—it is predominantly a function of arc current. Thus, prior artcontrol schemes that use arc power (or arc energy) to control theburn-off rate are complex and inaccurate. Example of such complex andinaccurate control schemes include: U.S. Pat. No. 4,866,247, entitledApparatus and Method of Short Circuiting Arc Welding, issued on Sep. 12,1989; U.S. Pat. No. 4,897,523, entitled Apparatus and Method of ShortCircuiting Arc Welding, issued on Jan. 30, 1990; U.S. Pat. No.4,954,691, entitled Method and Device For Controlling A Short CircuitType welding System, issued on Sep. 4, 1990; and U.S. Pat. No. 5,003,154entitled Apparatus and Method of Short Circuiting Arc Welding, issued onMar. 26, 1991. Some of these prior art patents teach control of thepower when a short is clearing by predicting the clearing of the short.They generally compare arc voltage or its first derivative to athreshold. However, the prior art attempts result in missed or falseshort clearing predictions.

Accordingly, a short circuit transfer welding power supply thatadequately controls the burn-off rate, preferably on a short-by-shortbasis, is desired. Preferably, the process should be controlled suchthat power is reduced when the short is clearing. Also, the power sourceused should be sufficiently fast to respond to the control, but notunduly expensive or limited in peak output current.

One of the causes of instability in a short circuit transfer weldingprocess relates to excessive pre-heating of the wire. Variations in thewire/puddle interaction caused by operator movement and/or changingpuddle geometry, can result in irregular pre-heating of the wire due toI²*R heat generation. Too much pre-heating of the wire can cause themelting rate of the wire to increase to a point where the molten ballgrows very quickly following the transition from a short to an arc. Thisquick melting, known as a flare-up, results in a rapid increase in arclength with a corresponding voltage increase.

The opposite extreme can also occur. If there is insufficientpre-heating of the wire, the short circuit frequency will increase assubsequent arc times become shorter. If energy is not added quicklyenough, the wire can eventually “stub” into the puddle. The end resultof such stubbing is either an explosive short clearing, or a sustainedshort circuit with no arc (sometimes called noodle welding). Over andunder preheating often occur in a cyclic fashion. Unfortunately, mostprior art controls adjust after a stub or flare-up has occurred. Forexample, when the control causes the heat to decrease to compensate forpast pre-beating, the process has already cycled to the under-heatingstage. Thus, the control actually exacerbates the problem.

Many of the problems with the aforementioned systems were addressed byU.S. Pat. No. 6,087,626, Method and Apparatus for Welding, Hutchison, etal., Jul. 11, 2000, hereby incorporated by reference. While that systemperforms much better than the prior art, a system that improves upon the'626 system is desirable. Accordingly, it is desirable to have a shortcircuit transfer welding process that more accurately compensates forthe pre-heating of the wire.

SUMMARY OF THE PRESENT INVENTION

According to a first aspect of the invention a method and apparatus forshort circuit welding includes providing welding power suitable forshort circuit welding, sensing the stick out length, and adjusting thewelding speed, such as wire feed speed or travel speed, adjusting awelding parameter, or adjusting thee gas mixture in response thereto.

Stick out is determined by measuring a welding parameter, and performingan FFT on the parameter, and then calculating stick out, in oneembodiment. Stick out can be either CTTWD or CPTPD.

According to a second aspect of the invention, a method and apparatusfor short circuit welding includes determining when a short is about toclear and calculating a value V_(c) defined by V_(c)=d/dt(kl*dp/dt), andcomparing V_(c) to a V_(threshold).

V_(threshold) varies in response to welding cycle history, including asa function, such as an average, of the time from crossing V_(threshold)to the short actually clearing in past welding cycles, in variousembodiments.

A comparison is made between dv/dt and threshold after V_(c) crosses aV_(threshold), and the controller determines that a short is about toclear in response to the comparison in another embodiment.

According to a third aspect of the invention, a method and apparatus forshort circuit welding wherein the waveform has at least a more rapidlyincreasing current segment that terminates at a termination currentmagnitude, and a less rapidly increasing current segment during a shortphase. The current magnitude that the rapidly increasing portionterminates at is reduced in response to welding cycle history.

The reducing is done in response to a function, such as an average, ofthe time from determining when the short was going to clear to the shortactually clearing in past welding cycles, according to variousembodiments.

According to a fourth aspect of the invention, a method and apparatusfor short circuit welding, wherein the current increases prior to theshort clearing, includes determining a value V_(c) defined byV_(c)=d/dt(kl*dp/dt), and comparing V_(c) to a V_(threshold), andinhibiting the comparison until current is increasing before the shortclearing.

According to a fifth aspect of the invention, a method and apparatus fordetermining the length of stick out, includes determining a desired massdeposition rate error and comparing the error to known values.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is block diagram of a system in accordance with the presentinvention; and

FIG. 2 is wave form in accordance with the present invention.

Before explaining at least one embodiment of the invention in detail itis to be understood that the invention is not limited in its applicationto the details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments or of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology and terminology employed herein is for the purpose ofdescription and should not be regarded as limiting. Like referencenumerals are used to indicate like components.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention will be illustrated with reference to apreferred control scheme, a preferred control circuit, a preferred powersource and illustrative waveforms, it should be understood at the outsetthat the invention is not limited to the components described herein.Other circuitry and control schemes may be employed while implementingthis invention.

A method and apparatus for controlling a short circuit (MIG) weldingprocess is described herein. A wire electrode is mechanically fed intothe weldment at a rate by a wire feeder in the short circuit transferwelding process. It is consumed into the weldment via a series ofalternating short circuit and arc events. This process is generallyreferred to as short circuit welding, or short circuit transfer welding.Generally, a welding machine used for short circuit welding includes atleast a power source, a controller and a wire feeder.

The short circuit transfer welding process is cyclical. One cycle of theprocess, as described herein, begins with the beginning of a steadystate arc, followed by a short circuit condition, and is completed withthe beginning of another steady state arc condition. A typical cyclelength is 10 msec. The electrode, and a portion of the base metal, aremelted during the short circuit transfer welding process by currentflowing through the electrode to the weldment. Generally, a portion ofthe wire material melts during the arc condition, and is transferredduring the short condition.

The preferred embodiment is generally implemented using the system ofU.S. Pat. No. 6,087,626. More specifically, FIG. 1 is a block diagram ofa short circuit transfer welding system that implements the presentinvention. Generally, a wire feeder 101 provides a wire 102 through awelding torch 104 to a weldment 103. A power source 105 provides powerto welding torch 104 and a workpiece 106. A controller 107 includes amicroprocessor (or a DSP or other integrated circuit in alternativeembodiments), and/or a discrete circuit. Controller 107 may be part ofpower source 105, part of wire feeder 101, power source 105 may have aseparate controller, or controller 107 may directly control the powerconverting of power source 105.

The preferred control scheme uses a current command signal to drive theoutput current. The command signal is comprised of multiple components.One component sets the long-term current command level (called thelong-term current command). Another component adjusts the currentcommand on a real-time or short-by-short basis (called theshort-by-short current command).

Arc voltage feedback is used to determine if the desired arc length ispresent and to adjust the long-term command on a short-by-short basis.The short-by-short current command is derived from real-time arc currentand voltage feedback (rather than power) and is used to control theburn-off rate by an instantaneous, or short-by-short, adjustment of thecurrent command. The preferred control scheme also uses a function ofthe time derivative of arc power (less the time derivative of arccurrent) to detect, in real time, when the short is about to clear.

The preferred embodiment uses a power source such as that shown in U.S.Pat. No. 6,329,636, Method and Apparatus for Receiving a Universal InputVoltage in a Welding Plasma or Heating Power Source (hereby incorporatedby reference) which has the capability to change its' output currentvery rapidly, on the order of 1000 amps/msec.

The wire burn-off rate is controlled by controlling the current on ashort circuit-by-short circuit basis (or period-by-period basis). Thisshort-by-short current control is combined with the current control setby arc voltage (to obtain a desired arc length). The power source andcontroller of the preferred embodiment are sufficiently fast to providethe desired current in much less than one weld cycle.

Thus, two control loops are in simultaneous use—an arc length loop usingarc voltage as feedback to set a long-term current command, and a wireburn-off loop using arc current and voltage as feedback to set ashort-by-short command. The two loops are weighted differently in thepreferred embodiment. Both arc voltage and arc current are used todetect, in real time, the short-clearing, and to terminate the process,as described below.

The waveform generally follows that of the '626 patent, and will not bedescribed herein except as necessary. FIG. 2 is a graph of current andvoltage in accordance with the present invention. A background currentflows during an arc phase. When a short circuit is established, thecurrent is commanded to a higher level along a rapidly increasingsegment. The rapidly increasing segment is followed by a plateau, whichis followed be a more slowly increasing segment. The plateau is omittedin some embodiments, or one or more cycles. When the short is about toclear the current is quickly lowered to a background level. After thearc is established the current is commanded to a high level. The highcurrent level during the arc phase is ended by decreasing the current asfast as possible to a plateau, and then after the plateau more graduallydecreasing the current to the background level. The plateau may beomitted in some embodiments, or for some cycles. The various currentmagnitudes and durations are controlled in a manner consistent with theprior art, except as set forth herein, to provide a robust andrepeatable process.

One aspect of the invention provides for the point at which the rapidlyincreasing segment ends to be adjusted due to past history of theprocess or welding cycle history. Welding cycle history, as used herein,includes parameters of past welding cycles, including functions thereof.More specifically, the time from when the indication of the shortclearing (dp/dt as set forth below) is received, until the short clears(precursor time) can be monitored. A running average can be used to endthe rapidly increasing segment earlier or later. Also, it can be endedearlier when a prior short (or shorts) cleared before the current couldbe lowered. The end of the segment can be based on time, current,voltage, power, or functions thereof crossing a threshold. Also, the,overall process can be controlled using precursor time as one of thefeedback parameters.

As is known in the art, it is desirable to reduce the current prior tothe short clearing. The present invention uses more information than canbe obtained from the voltage waveform alone to quickly and consistentlydetect the imminent short clearing. More specifically, controller 107uses, in one embodiment of the invention, the second derivative of thepower to detect the short clearing event, in real time. Controller 107solves an equation V_(c)=d/dt(dP/dt), in real time. When V_(c) crossesV_(threshold) controller 107 determines, in real time, that the short isabout to clear. Alternatives includes using other functions of dP/dt,using functions of dV/dt instead of or with dP/dt, as well as usingdR/dt, or higher order derivatives of these parameters, or otherfunctions of these parameters, and combinations thereof. The equationmay be implemented with discrete circuitry, or using a microprocessor,DSP, etc, in a well known manner.

According to another alternative V_(threshold) is varied in response topast history of the process. For example, in one embodiment, a runningaverage of the time between the crossing the V_(threshold) and theactual short clearing for a number of weld cycles is calculated and usedto adjust V_(threshold). Other functions of the time are used in otherembodiments.

Avoiding false short detections can be enhanced by monitoring thewelding parameters after V_(threshold) is crossed. The inventor hasdetermined that when a valid dP/dt is determined, the voltage waveformis either flat or slightly decreasing because the resistance increasedue to the necking down of the of the molten bridge is greater than thedecrease in voltage due to falling current. However, this is not truefor a false positive dP/dt. Thus monitoring dV/dt for a negative slopeafter V_(c) has been crossed can indicate a false positive. Accordingly,the controller indicates a short is about to clear when V_(threshold) iscrossed and the current is abruptly reduced, and when dV/dt is notnegative.

According to yet another embodiment, the detection of the short about toclear is inhibited until the process is close to where the short willclear. For example, it can be inhibited until the process reaches thestart of the rapidly increasing segment, the end of the rapidlyincreasing segment, or after the plateau of the short phase.

It is known that a long arc time results in a long transit time of thewire back to the puddle. During this transit time, the current is low,and therefore, the I²*R heating at the contact area in the contact tubeis low. This produces a relative cold spot in the wire which begins totravel toward the puddle. As this cold region of wire approaches theweld puddle, the size of the molten ball formed after the short clears,decreases. Also, the time spent in the arc mode decreases. This shift intime from the arc to short circuit increases the overall I²*R heating ofthe wire. This increased I²*R heating produces a localized hot spot inthe wire near the contact tube, bringing the cycle back to thebeginning. Thus, this process may be cyclic in nature.

The frequency of this cyclic phenomenon is related to a number offactors. Chief among these are the stick out length, or more preciselycontact point to puddle distance (CPTPD). Contact point, as used herein,is the point at which electrical contact is made between the outputpower and the welding wire. Prior art schemes, to the extent theyconsidered stick out, used contact tip to work distance (CTTWD). Thesystem described herein will be described with reference to stick out,CPTPD, or CTTWD, since, while CTPTD is most desirable, the invention maybe implemented, albeit less effectively, using CTTWD.

The fundamental frequency of oscillation of the cyclic process may beobtained by performing an FFT on an welding parameter such as current,voltage, power, or functions thereof. Welding parameter, as used herein,includes output current, voltage, power, welding speed, etc. Given thefrequency from the FFT, the CPTPD may be determined as the inverse ofthe transit time of a section of wire equal to the length of the stickout, traveling at a velocity equal to the wire feed speed. It should benoted that higher modes of this fundamental frequency could conceivablybe excited.

A desirable process provides for a constant CPTPD or stick out. Giventhe ability to sense the stick out quickly and accurately, the processmay be controlled to maintain a constant CPTPD. For example, weldingspeed (more particularly wire feed speed or travel speed) can beadjusted in response to the calculated stick out or CTPTD. Weldingspeed, as used herein, includes the wire feed speed and the travel speedof the welding gun. Also, the shielding gas mixture can be controlled inresponse to CPTPD.

Additionally, welding parameters may be controlled in response to CTPTD.A preferred control provides for controlling welding current in responseto CPTPD, preferably determined using an FFT of a welding parameter.Another alternative provides for determining stick out by comparing themass burn-off rate to a desired mass burn off rate, and determining anerror. The error is compared the error to known values to determine thestick out, because stick out is dependent on the mass burn-off rateerror.

Numerous modifications may be made to the present invention which stillfall within the intended scope hereof. Thus, it should be apparent thatthere has been provided in accordance with the present invention amethod and apparatus for short circuit welding that fully satisfies theobjectives and advantages set forth above. Although the invention hasbeen described in conjunction with specific embodiments thereof, it isevident that many alternatives, modifications and variations will beapparent to those skilled in the art. Accordingly, it is intended toembrace all such alternatives, modifications and variations that fallwithin the spirit and broad scope of the appended claims.

1-27. (canceled)
 28. A method of short circuit welding that includesdetermining when a short is about to clear; the improvement comprisingcalculating a value V_(c) defined by V_(c)=d/dt(kl*dp/dt), wherein kl isa scalar, dp/dt is the derivative of the output power, and comparingV_(c) to a V_(threshold).
 29. The method of claim 28 whereinV_(threshold) varies in response to welding cycle history.
 30. Themethod of claim 29 wherein the V_(threshold) varies in response to afunction of the time from crossing V_(threshold) to the short actuallyclearing in past welding cycles.
 31. The method of claim 30 wherein theV_(threshold) varies in response to an average of the time from crossingV_(threshold) to the short actually clearing in past welding cycles. 32.A method of short circuit welding that includes determining when a shortis about to clear, the improvement comprising: comparing dv/dt to adv/dt threshold after V_(c) crosses a V_(threshold), and wherein thecontroller determines that a short is about to clear in responsethereto.
 33. A method of short circuit welding that includes determiningwhen a short is about to clear, and providing at least a more rapidlyincreasing current segment that terminates at a termination currentmagnitude, and a less rapidly increasing current segment during a shortphase, the improvement comprising: reducing the termination currentmagnitude of in response to welding cycle history.
 34. The method ofclaim 33 wherein the reducing is done in response to a function of thetime from determining when the short was going to clear to the shortactually clearing in past welding cycles.
 35. The method of claim 33wherein the reducing is done in response to an average of the time fromdetermining when the short was going to clear to the short actuallyclearing in past welding cycles.
 36. A method of short circuit weldingthat includes determining when a short is about to clear, and whereinthe current increases prior to the short clearing, the improvementcomprising, determining a value V_(c) defined by V_(c)=d/dt(kl*dp/dt),and comparing V_(c) to a V_(threshold), and inhibiting the comparisonuntil current is increasing before the short clearing.
 37. A method ofdetermining the length of stick out, comprising the steps of:determining a desired mass deposition rate error; and comparing theerror to known values.
 38. A welding controller that determines thelength of stick out, comprising: means for determining a desired massdeposition rate error; and means for comparing the error to knownvalues, connected to the means for determining.
 39. The method of claim28, further comprising determining a rate of change of the output power,and determining when the short is about to clear in response to thepower delivered.
 40. A short circuit welding power supply, comprising apower circuit and a controller connected to the power circuit, whereinthe controller includes means for determining when a short is about toclear by calculating a value V_(c) defined by V_(c)=d/dt(kl*dp/dt),wherein kl is a scalar, dp/dt is the derivative of the output power, andcomparing V_(c) to a V_(threshold).
 41. The apparatus of claim 40further comprising means for varying V_(threshold) in response towelding cycle history.
 42. The apparatus of claim 41 wherein the meansfor varying includes means for varying V_(threshold) in response to afunction of the time from crossing V_(threshold) to the short actuallyclearing in past welding cycles.
 43. The apparatus of claim 41 whereinthe means for vary includes means for varying V_(threshold) in responseto an average of the time from crossing V_(threshold) to the shortactually clearing in past welding cycles.
 44. The apparatus of claim 40,further comprising means for determining a rate of change of the outputpower, and determining when the short is about to clear in response tothe power delivered.
 45. A short circuit welding power supply comprisinga power circuit and a controller connected to the power circuit thatincludes means for determining when a short is about to clear, andfurther comprising means for comparing dv/dt to a dv/dt threshold afterV_(c) crosses a V_(threshold), and wherein the controller determinesthat a short is about to clear in response thereto.
 46. A short circuitwelding power supply comprising a power circuit and a controllerconnected to the power circuit that comprises means for determining whena short is about to clear, and means for providing at least a morerapidly increasing current segment that terminates at a terminationcurrent magnitude and a less rapidly increasing current segment during ashort phase, and further comprising means for reducing the terminationcurrent magnitude of in response to welding cycle history.
 47. Theapparatus of claim 46 wherein the means for reducing includes means forreducing in response to a function of the time from determining when theshort was going to clear to the short actually clearing in past weldingcycles.
 48. The apparatus of claim 46 wherein the means for reducingincludes means for reducing in response to an average of the time fromdetermining when the short was going to clear to the short actuallyclearing in past welding cycles.
 49. A short circuit welding powersupply comprising a power circuit and a controller connected to thepower circuit that comprises means for determining when a short is aboutto clear, and means for increasing the current prior to the shortclearing, and further comprising means for determining a value V_(c)defined by V_(c)=d/dt(kl*dp/dt) and comparing V_(c) to a V_(threshold)and inhibiting the comparison until current is increasing before theshort clearing.